SWAN LAKE INTEGRATED WATERSHED MANAGEMENT PLAN GROUNDWATER RESOURCES AND ISSUES Swan Lake Integrated Watershed Management Plan Groundwater Resources and Issues Introduction Groundwater forms the primary source of rural, municipal and industrial water supply in the Swan Lake watershed. It also provides base flow to rivers and creeks and is an important component in sustaining many of the area’s wetlands. Several extensive bedrock aquifers exist but in parts of the watershed the groundwater quality in these aquifers is saline or deteriorates rapidly with depth. Surficial sand and gravel aquifers are also present in some areas but, while some of these aquifers are very productive, most appear to have limited extent. This summary of the geology and hydrogeology of the watershed is taken primarily from previous studies. Regional groundwater mapping of the Swan Lake 1:250,000 map sheet area was conducted by Betcher (1991) and mapping of the Duck Mountain 1:250,000 map sheet was done by Little and Sie (1976). Rutulis (1984) discussed the hydrogeology of the Swan River Formation in Manitoba. There have been a number of studies of the salt springs found in the northern part of the watershed including Cole (1949) and van Everdingen (1971). The surficial geology of the area was mapped by Nielsen (1988). The bedrock geology is summarized on a 1:1,000,000 map prepared by the Manitoba Mineral Resources Division (1979). Geology The topography of the western part of the watershed is dominated by the Duck Mountain and Porcupine Hills upland areas which are separated by the Swan River valley. The steep easterly slopes of the uplands grade to the west into a relatively flat lying plain known as the Westlake Plain. The Swan River valley is thought to have been formed by an eastward flowing river during the Tertiary (about 65 million to 2 million years ago). The youngest bedrock units in the watershed are found beneath the upland areas of the Porcupine Hills and Duck Mountain on the west and extend to the base of the eastern escarpments of these uplands (see Figures 1 and 3). These rocks consist primarily of relatively soft bentonitic shale. As the land surface elevation decreases to the east, bedrock is composed of progressively older sediments (the youngest sediments lie at the top of the rock sequence with older sediments below) although shale remains the predominant lithology. Near the base of the escarpments the bedrock lithology changes to an interlayered sequence of sandstone and shale which extends further to the east. This geologic unit is known as the Swan River Formation and forms an important aquifer in the watershed. The Swan River Formation is underlain by much older bedrock composed of limestone, dolostone and shale. These rocks form the bedrock surface in the vicinity of Swan Lake. The bedrock geology is shown in plan view in Figure 1 and in an east- west cross-section in Figure 3. Note that a few test holes in the upland areas have intersected sandy to silty sediments lying at the till/bedrock contact. These are believed to be remnants of the Wynyard Formation which was deposited during the Tertiary. The extent of these sediments is unknown. Bedrock is overlain by a variable thickness of overburden throughout the watershed, although there are a few very local areas where bedrock is found at ground surface. Overburden thickness varies from being absent where bedrock outcrops occur to as much as 300 meters or more below parts of Duck Mountain and the Porcupine Hills. The overburden is primarily composed of till (see Figure 2) which was deposited during a series of glacial advances and retreats over the past 2 million years. Till consist of unsorted mixtures of clay, silt, sand, gravel and boulders. Within or lying on the till we find scattered lenses or layers of sand and gravel which formed as outwash deposits in areas where large volumes of water flowed from the nearby glaciers. Somewhat younger sediments, related to glacial Lake Agassiz which formed behind the most recent ice sheet as it retreated to the north and a second glacial lake which formed in the Swan River valley, are found overlying the till in the topographically lower parts of the watershed, particularly in the Swan River valley. These are mostly lacustrine (formed within lakes) sediments consisting of flat lying layers of clay, silt and sand. Sand deposits are also found along the lower flanks of Porcupine Mountain and south of Swan Lake. The sand deposits are ancient beaches that formed at the edges of Lake Agassiz as the lake level rose and fell. The youngest unconsolidated deposits, consisting of alluvial and organic materials, overlie the glacial and lacustrine sediments west of Swan Lake. See Figure 2 for a map of the surficial geology of the watershed and adjacent areas. Hydrogeology The geologic units discussed above can be divided into two hydrogeological classes: aquifers or aquitards. An aquifer is a geologic unit that can provide useful quantities of water to wells completed in the unit while an aquitard is less permeable and will not provide useful quantities of water to wells. In the Swan River watershed we find aquifers formed both by bedrock and overburden units. Bedrock aquifers consist of limestone and dolomite forming what is known as the Carbonate aquifer and by sandstones of the Swan River Formation which are termed the Swan River aquifer. Shale is generally not sufficiently permeable to form an aquifer although in some parts of the watershed the shale is fractured and low-capacity wells may be installed. Typically though, we would consider shale to be an aquitard. Sands or sand and gravel layers or lenses in the overburden form aquifers, although a sand or sand/gravel unit must be saturated throughout the year and sufficiently thick to form a usable aquifer. None of the overburden sand/gravel units has been formally identified with an aquifer name. Clays, tills and organic deposits are considered aquitards. Carbonate Aquifer The Carbonate aquifer directly underlies surficial materials in the area surrounding Swan Lake (see Figure 1) but also lies within typical well construction depths beneath the Swan River aquifer in the Westlake Plain, much of the Swan River valley and the lower parts of the Manitoba Escarpment. Water well records have been submitted for only a few dozen wells drilled into the aquifer in this watershed (see Figure 4). Reported well yields range from about 1 gpm to 40 gpm. There is little known about where recharge occurs to this aquifer or where it discharges. Groundwater quality in the Carbonate aquifer is poor throughout most of its extent with total dissolved solids (TDS) contents typically exceeding 1000 mg/L and increasing to 3,000+ mg/L in the Swan River valley. Groundwaters with a TDS content exceeding about 1000 mg/L are generally undesirable as a drinking or household water source; groundwater with a TDS content above 2000 mg/L will likely have a salty taste in this area. It is expected that water quality in the aquifer will decline significantly with depth although there is little sampling information to support this. Numerous salt springs occur near Dawson Bay and on the west side of Swan Lake. These springs discharge directly from exposures of the aquifer or in areas where the aquifer is covered by shallow overburden. It is believed that the springs are associated with ancient reef structures in the bedrock which allow upwelling of deeper saline waters. The salt springs have salinities near to that found in sea water or even higher and are remarkable natural features, little known outside the local area. Water from some of these springs or from shallow wells dug near the springs was evaporated and the residual salt shipped south by York Boats and Red River carts to the settlement areas near Portage and Winnipeg from about 1818-1874. Swan River Aquifer Sandstone beds in the Swan River Formation form the most utilized aquifer in the watershed and surrounding areas. The aquifer extends east and north from the Manitoba Escarpment and underlies the Swan River valley (Figure 1). The aquifer is also accessible at typical well construction depths from the lower parts of the Escarpment. The Swan River aquifer is the source of water for several hundred farm, business and rural properties in the watershed including the Louisiana Pacific OSB plant (Figure 4). It is also the water source for municipal water systems supplying the Town of Minitonas and the village of Bowsman. Regional studies indicate groundwater flow is from the topographic highs formed by the Porcupine Hills and Duck Mountain toward the topographic lows formed by the major lakes. The Swan and Woody Rivers are thought to be significant discharge areas as outcrops of the aquifer have been mapped along these rivers. Yields from wells completed into the aquifer are quite variable, depending on the thickness, grain size and cementing of the sandstone layers intersected by the well. Reported yields range from about 1 gpm to 100 gpm with a mean of about 20 gpm. Groundwater quality is also quite variable. Total dissolved solids concentrations in analyses available in the Water Stewardship data base range from 316 mg/L to 15,000 mg/L (Figure 5). Most wells produce groundwater with a TDS in excess of 1000 mg/L. As discussed previously, groundwaters with a TDS in excess of 1000 mg/L are generally undesirable from an aesthetic standpoint. Groundwaters frequently contain elevated sodium and sulphate concentrations although these groundwaters may be quite “soft” and desirable for household uses since a water softener is not needed. Spatial trends in water quality are not clear – there is an expectation that water quality should deteriorate with depth in the aquifer but this has not been well established.